1. Field of the Invention
The present invention relates to a method of manufacturing a glass substrate for a magnetic disk device such as a hard disk device.
2. Description of the Related Art
Recently, a large-capacity hard disk device is mounted in an information terminal such as a personal computer, a car navigation system, a mobile phone, and a portable music player, and a magnetic disk capable of performing high-density recording is used for this type of large-capacity hard disk device. As a substrate for the magnetic disk capable of performing high-density recording, a glass substrate having excellent surface smoothness and high rigidity is used. The glass substrate has a disc shape with a circular hole at the center, that is, an annular shape. However, if an edge portion on the internal circumference or external circumference has a sharp shape with an angled edge, cracking is likely to occur and its mechanical strength is degraded. Therefore, chamfering of the edge portion is applied.
The chamfering is performed, for example, by a method described in Japanese Patent Application Laid-open No. 2003-231044. However, in the conventional method, chipping occurs from a chamfered portion toward a main surface, which is a recording surface of the glass substrate.
FIG. 21 is an explanatory diagram of chipping that occurs on the glass substrate, which is an enlarged cross section of an end portion of a glass substrate 7. As shown in FIG. 21, chipping C has occurred with a length of C1 and a depth of C2 from a chamfered portion 77 toward a main surface 71 of the glass substrate 7. Reference numeral 73 denotes an outside surface of the glass substrate 7, and reference numeral 79 denotes the surface of an ideal chamfered portion. The chipping occurs due to wear (line, collapse of shape) of a groove on a grindstone for chamfering or degradation of rotation accuracy of the grindstone and the glass substrate at the time of chamfering. As the thickness of the glass substrate becomes thinner and the surface state thereof becomes mirror-finished surface, chipping is likely to occur, and a large-size chipping depth is equal to or more than 100 μm.
As a method of suppressing the chipping, for example, an ELID grinding method (Electrolytic in-process dressing method) described in Japanese Patent Application Laid-open No. 2001-105292 has been proposed. According to this method, however, even if the chipping size can be decreased, an ELID electrode and a voltage applying unit are required, which are expensive. Further, a machining allowance increases, and because a fine high-count grindstone is used, a machining time becomes too long, which is not practical.
Under such a background, a conventional method of manufacturing the glass substrate for the magnetic disk includes, as described in Japanese Patent Application Laid-open No. 2005-225713, a first lapping step of forming a glass preform having a thickness several times thicker than a thickness of a finished product according to a method such as press-forming or plate-glass forming method, and sequentially lapping a main surface of the glass preform; an inside and outside diameter-coring step of forming an annular glass substrate by coring; and an inside and outside edge-chamfering step of performing grinding of inside and outside surfaces and chamfering of an edge portion. Further, an inside and outside surface-polishing step is performed for polishing the inside and outside surfaces, and thereafter, at a second lapping step, the thickness is adjusted to the thickness of the finished product, and the surface of the glass substrate is lapped considerably to remove chipping, and then, polishing is performed for a plurality of times, to obtain a mirror finished surface of the glass substrate.
On the other hand, as a method of preventing occurrence of the chipping at the time of forming the annular glass substrate by coring, a method of performing the coring in a state in which a plurality of glass substrates are overlapped and integrated is disclosed in Japanese Patent Application Laid-open No. H6-198530.
In the conventional method, however, because the surface of the glass substrate is lapped considerably to remove the chipping, investment for a lapping device is required, and loss of the glass material increases, and the process time increases, thereby causing a cost increase in manufacturing the glass substrate. Further, even if the method of performing the coring in the state in which a plurality of glass substrates are overlapped and integrated is used, occurrence of the chipping cannot be prevented sufficiently at the time of chamfering at the subsequent step. Therefore, a lapping for removing the chipping is required, thereby causing the same problem as above.